Control valve

ABSTRACT

A control valve is provided having a control structure therein in which dissipation of liquid pressure energy is achieved by the use of two matched and opposed submerged jets that have substantially equal mass flow which result is achieved by utilizing a wall that defines an interior chamber with the matched jets physically comprising 180° opposed orifices through the wall and the control means simultaneously adjusts the area of the orifices.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division of my earlier filed application Ser. No. 687,924,filed May 19, 1976, now U.S. Pat. No. 4,103,696, which is acontinuation-in-part of my application Ser. No. 408,096, filed Oct. 19,1973 now abandoned.

BACKGROUND OF THE INVENTION

In control valve structures that have been used in the past, one of themajor problems that has presented itself has been cavitation thatresults on the outlet port of a normal control valve due to the pressuredrop through the valve. In the hydraulics of a normal valve there isliquid flowing to the valve which is under pressure and the valve, ineffect, presents a throttling device in the flow stream. As the liquidpasses through the valve structure, the head energy of the flowing massis converted to kinetic energy and when this conversion takes place athigh pressure dissipation, the static pressure of the liquid drops to apressure less than the vapor pressure of the liquid and this causescavitation and flashing of the liquid into a gaseous state which oncollapse represents damage to the valve parts and the downstream piping,together with noise and vibrational effects which are well known tothose skilled in the art. See, for example, Stiles, G. F., November1961, "Cavitation in Control Valves", Instruments and Control Systems,Volume 34, No. 11. In the art there are several designs to achieve somecontrol of cavitation. One of the forms that the prior art valves takeis, for example, stepped cone valves, which present a frictional loss tothe liquid stream as it passes therealong and further gives a multiplevelocity change throughout the steps. Another form uses abradedapertures through a valve plug, such as shown in the Curran U.S. Pat.No. 2,918,087. Still other forms are multiple valve plugs rather thanutilizing a single plug in a structure which, in effect, creates aplurality of restricting orifices and thus takes the pressure drop in aseries of small steps. Still a further approach is to utilize fluidvortexes within a plurality of cascaded stages where the fluid is turned90° as it passes through from one stage to the next and spirals aroundthe plug. In all of these prior art valves, considerable expense isrequired to produce them, and in some cases they do not controlcavitation to a degree where it can be beneficial.

SUMMARY OF THE INVENTION

It has been found that by placing two adjustable area jets that have anequal mass flow in 180° opposition and in a specific spacing between thejet forming means, that the hydraulic energy of the jets can beconverted into heating of the fluid by a stable eddy formation in arestricted volume of space. In a given valve structure the spacing ofthe jet forming means is a fixed dimension and it is therefore necessaryto provide means for allowing this fixed dimension to give properdissipation at different valve openings and head dissipations. Ineffect, the twin jets that come out of the opposed orifices impact inthe center of the valve and form an ellipsoidal dissipating pressurevolume. The pressure at the center of this area is at substantially theinlet pressure, and by properly spacing the opposed orifices, the venacontracta that is created by the jet discharge will be maintainedadjacent to the surface of the ellipsoid. The stable decreasing velocitygradient within the ellipsoid dissipates the pressure energy, and thesmall cavitation zone that is present is extremely short in length sincethe cavitation zone is effectively from the vena contracta downstreamthereof, and inasmuch as the vena contracta is close to the ellipsoid,these zones become very short. Furthermore, the velocity of the jet atthe vena contracta location is very high and can be shown to beapproximately equal to 12.2 times the square root of the pressuredifferential. Therefore, the time for cavitation voids to grow isextremely short.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a complete valve made in accordancewith the principles of the invention;

FIG. 2 is a central sectional view taken substantially on lines 2--2 ofFIG. 1;

FIG. 3 is a perspective view of a typical arrangement for anotherembodiment of the invention;

FIG. 4 is a central sectional view of another embodiment;

FIG. 5 is an elevational sectional view of another embodiment and

FIGS. 6, 7 and 8 are graphical representations of characteristics ofvalve operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In proceeding with the invention, there is provided a valve body havingan inlet port and a pair of outlet ports so that the valve may be usedas a 90° valve or a straight in line valve depending upon which of theoutlet ports is sealed by a plug. Also in the drawings a handwheeloperator is illustrated, but it should be understood that other types ofoperators may be utilized as well known to those skilled in the art.

With reference specifically to FIG. 1, the valve body generallydesignated 10 comprises a cast member with bosses 11, 12 and 13extending therefrom to provide inlet port 14 and outlet ports 15 and 16.For convenience in manufacture, the body 10 is cast with a substantiallyhollow interior having a substantially circular interior wall 18 whichis open at its top where a threaded opening 20 may be provided and intowhich a cover 22 may be screwed with a sealing gasket means 23. Withinthe cover 22 is located a valve actuator stem 24 which is suitablesealed therein with a packing at 25 and which forms an actuating meansfor orifices 30, 31, as will presently appear. Substantially midway ofthe wall 18 and in continuation of inlet boss 11 is a circularenlargement having inner wall 26 forming a passage 27 which terminatesat a wall 28 which, as can be seen in FIG. 2, provides a passage to theoutlet port 15 or 16.

Preferably a piece of soft stainless steel tubing 29 forms a wall meansand is punched with a pair of opposed orifices or apertures 30, 31.These apertures can take a variety of shapes and are shown herein asbeing somewhat triangular in outline shape to give the desired linearvalve characteristic with respect to stem travel. This tubing 29 may beheld in proper orientation position by means of a locating pin 32 asseen in FIG. 1. The O D of the tubing is sealed to the interior wall 18of the valve body by two static O-rings 34, 36. The inlet flow throughthe port 14 is divided into two equal parts as seen in FIG. 2, whichparts form symmetrical flow paths and are designated by the arrows 38,39, and is channeled around the passage 27 so that it will enter theorifices or apertures 30, 31 as seen by the arrows 41, 42 and bedirected to the chamber formed by the wall means 29.

The flow control means is formed by a slidable tubing member 46 which isinserted into the internal diameter of the tubing member 29. At itsupper boundary an O-ring seal 48 is provided and a pair of ears 50 crossthe upper end of the flow control means tubing 46 so that attachment tothe lower end 52 of the actuator stem 24 may be effected. At the lowerend of the tubing 29 there is provided a form of a seat which can be aplastic ring seat 52 held in place by a press fit bushing 53 oralternately a hardened or ground seat if high temperature applicationsare envisioned for the use of the valve of this invention. In thisfashion bubble tight shutoff on the valve closure can be achieved oncethe flow control means tube 46 is completely in a down direction andengaging the seat.

It will be noted in the diagrammatic representation in FIG. 2 that afterthe flow passes through the apertures or orifices 30 and 31, that jetswill be formed by the fluid as it passes through the orifices and thesejets will form a vena contracta as at 60 and 61 and will combine anddissipate the differential pressure in a dissipation zone shown by theelliptical line 62 in FIG. 2. The pressure at the center of theellipsoid 62 is substantially the inlet pressure even though this pointis downstream of the throttling orifices, and it has been found that byproper spacing of the orifice apertures 30 and 31 that the venacontracta at 60 and 61 may be made adjacent the surface of the ellipsoid62. It has been found in practice that the ideal spacing in order toachieve this particular result of the apertures or orifices 30, 31 issubstantially within the range of -5% to +30% of a distance S defined bythe expression [1.3+0.1∛ΔP]d where ΔP=the maximum design dissipationpressure in pounds per square inch and d is the inlet supply portdiameter in inches.

The exit flow from this valve for in-line ports is both upward as viewedin FIG. 1 and downward as designated by the arrows 70 and 71 intochamber 69 formed by the wall 28 where the flow will be re-combined andproceed outwardly as indicated by the double arrow 72. The exit flow forangle ports as viewed in FIG. 1 is both upward and then downward throughthe vertical passageway as shown by arrows 71 and 73; and downwardthrough the center valve section as shown by arrow 74.

It will be seen that with the use of two matched jets which generateequal flow rates a stable dissipation area can be formed. The use of aplurality of jets destroys the stability. If nozzles are substituted forthe orifice configuration, it will be appreciated that differentspacings will be necessary to achieve the same results.

With a valve of the configuration described above, a number of desirablefeatures are attained. Referring to FIG. 6 of the drawings, there isillustrated the flow characteristics of a valve of three different typesin which the amount of flow through the valve has been diagrammed as apercentage of stem travel. Normally a characteristic is selected to suitthe particular application to obtain the type of control desired. Theflow characteristic of valve flow coefficient C_(v) is expressed as theratio of flow in gallons per minute divided by the square root of thedifferential pressure over the specific gravity of water or the fluidwhich is passing through the valve. Now if we plot the flow as againstthe square root of the differential pressure (see FIG. 7), we can obtainan idea of the valve's liquid flow characteristics and coefficientC_(v). It has been established in tests that a departure from a straightline relationship between the liquid flow rate and the square root ofthe differential pressure indicates cavitation. FIG. 7, in effect,represents a four-inch Vee Ball valve where the beginning of the curveis a straight line that corresponds to the valve flow coefficient ofthis particular valve. When the square root of the differential pressurereached 3.75, as illustrated by the first broken line, the curve beganto veer off to the right indicating that cavitation had begun. There area number of dimensionless ratios which can be used to describe pointswhere cavitation begin. K_(c) which is sometimes known as the cavitationindex is used to describe the point of initial departure from aproportional relationship as seen in FIG. 7 and is a ratioexperimentally determined. It is defined as the ratio of the pressuredifferential across the valve at a point of deviation from a straightline divided by the difference between the valve inlet pressure and theliquid vapor pressure. There is still another dimensionless ratio calledrecovery coefficient which is also experimentally determined inidentifying the point above which no increase in flow rate is achievedfor an additional increase in pressure drop. In effect, it defines whatis termed in the art as a fully choked flow condition and can be thoughtof as a measure of the lost pressure converted to velocity at the venacontracta that will be recovered at the valve outlet. The coefficientK_(m) is defined as the differential pressure at the intersection of thestraight lines where Q is proportional to the square root of thepressure differential (see the second broken line in FIG. 7) and theline where flow becomes a constant divided by the difference between thevalve inlet pressure and the vapor pressure of the liquid.

Comparing the performance of the instant invention with known valvesrenders results as shown in FIG. 8. In FIG. 8 there is a plot of therecovery coefficient or the cavitation index against the pressuredifferential divided by the difference between the valve inlet pressureand the vapor pressure of the liquid. As will be seen, the instant valveevidences superior test performance as compared to other valve designsof the prior art, the most exemplary of which are the special cavitationresistant designs followed respectively by globe valves, angle valvesand other types. Additionally test results of the flow rate to stemtravel yield the broken line curve as seen in FIG. 6 where the valveunder test utilized 60° triangular orifices 30 as seen in FIG. 1 of thedrawings.

Referring to FIGS. 3, 4 and 5 of the drawings, there is illustratedapplication for liquid control utilizing the principles of the instantinvention. In an application such as a dam discharge as seen in FIG. 3,there is a requirement to dissipate full inlet pressure all the way downto atmospheric. Such a head loss situation creates full cavitation ofthe flow downstream of the control valve. To this end, a structureconsisting of two opposed knife gate valves submerged in the downstreamwater and being fed by symmetrical flow paths may be used. Referring toFIG. 4 there is shown a flange 11' which couples a pair of divergingpipe sections having inlet flow passage 14' that go into passages 38'and 39' and thence to control gate valves 30', 31' which are spacedapart a distance S as in the previous embodiment. The two symmetricalflow paths 38' and 39' should have a diameter of approximately 0.7 ofthe main inlet diameter 14' and equal length so that equal head loss inthe liquid flow to valves 30', 31' is had. Each of the knife gate valvesshould be identical and by way of example in FIG. 5 there is illustrateda typical knife gate valve which is equipped with a V notch flowregulator insert 70, the knife gate 71 being of the complementary shapeand structure and being operated by a stem 72 and for simplicity a handwheel 73. It should be understood, however, that the valve openingsshould be identical at any one time for both valves 30' and 31', andtherefore, the operators or actuating means such as the hand wheel 73should be tied together as shown by the dotted lines 74 so that amatched motion may be had. It would be understood by those skilled inthe art that suitable operators and remote control positioners would beutilized in a particular application. Illustrated by broken line 75 inFIG. 5, there is diagrammed the water level of the lake, stream or whathave you downstream of the dam, for example, as seen in FIG. 3, and inorder for this application to work properly, it has been found that thesubmergence of the valves 30' and 31' as seen in FIG. 5 should be equalto 2S, that is twice the orifice spacing distance with no maximumsubmergence dictated except for a suitable clearance above the bottom ofthe lake or pond which should be equal to at least 2S.

The above arrangement as seen in FIGS. 3, 4 and 5 entails the use of two180° closed jets of equal mass flow rates with the jet forming meanspositioned at the prescribed separation S. Jet forming means is therestricting orifice as has been known in the art. Since cavitationcavities initiate at the vena contracta of the jet, and since the venacontracta is in a liquid filled space away from all physical elements,the impingement of these two opposed jets will stall the high jetvelocity, recovering the velocity energy and convert it to a positivepressure. Any cavities initiating in the vena contracta are forced tocollapse in the liquid filled space between the two orifices ensuringthat no cavitation damage can occur to any valve element.

Typical parameters for the arrangement shown in FIG. 3 would be a12-inch supply with two 8 inch branches and terminated in 8 inch knifegate valves spaced 21 inches apart and at least 42 inches below theliquid surface of the discharge basin. The invention testing has shownthat devices can be produced that are free from cavitation damage,undesired choking, and freedom from plugging with particulate matter.The sharp-edged orifices as used in the valve of the invention havedemonstrated long life capability even on light slurry service bymaintaining an initially sharp edge over a long period of time whichresults in the stabilization of loss characteristic of the valve at anyset opening over extended usage.

I claim:
 1. A liquid control valve comprising a casing having inlet andoutlet ports, wall means in said casing having a single pair of opposedspaced orifices formed therethrough, said wall means defining a chamberwith an open top and bottom and unobstructed space between the opposedorifices, means directing flow from the inlet port to said orifices,second means directing flow from the top and bottom of said chamber tosaid outlet port, and flow control means fitted to simultaneouslycontrol the size of said pair of orifices so that each has equalcross-sectional area, said orifices directing jets at each other andbeing spaced apart a distance to create a stable dissipation zone withinsaid chamber and discharging the flow from the top and bottom of saidchamber with substantially spherical expansion.
 2. A valve as in claim 1wherein the orifices are spaced apart substantially within the range of-5% to +30% of a distance defined by the expression

    [1.3+0.1∛ΔP]d

where ΔP equals the maximum design dissipation pressure (PSI) and d isthe inlet supply port diameter in inches.